Hypertension in pregnancy is associated with increased maternal and fetal morbidity and mortality. Hence, its early detection and management is essential for a good perinatal outcome. It is important to recognize that the mean arterial blood pressure at any given time is the resultant of the momentary interaction of a number of determining factors: the pumping action of the heart, cardiac output (heart rate multiplied by stroke volume), peripheral resistance (which depends on the caliber of the small arterial vessels); viscosity of the blood; quantity of blood in the arterial system; and the elasticity of vessel walls.
Measurement of blood pressure per se does not provide information about the role that each of the above causal factors may have played to determine its final magnitude. Such additional information is of importance when attempting to correct abnormal cardiovascular situations such as severe hypertension, hypotension, cardiac failure, or excessive blood loss. However, in the majority of noncatastrophic clinical situations, mean arterial blood pressure (MAP) is primarily determined by the pumping action of the heart, heart rate (HR), peripheral resistance, and the elasticity of vessel walls (since blood volume and viscosity are usually unchanged).
Currently, noninvasive techniques utilizing the carotid artery pulse have been described for assessing myocardial contractility, on which the pumping action of the heart is primarily dependent. These measurements, which are known as "systolic time intervals," are derived from simultaneous recordings of the electrocardiogram, phonocardiogram, and carotid artery pulsations and they provide a basis for indirect determination of total electromechanical systole.
While systolic time interval measurements offer a physiologic approach to cardiovascular assessment, they are not widely used due to technical difficulty in obtaining a noninvasive carotid artery pulse, unless patient movement is restricted or during short periods of breath-holding. Despite these practical problems, the physiologic basis for the use of systolic time intervals is fundamentally sound and provides, in principle, a method for assessing myocardial performance.
FIG. 14 shows carotid artery pulse wave showing contour changes with increasing age and hypertension/atherosclerosis. RET measurements also are shown.
FIGS. 1B and 1C show cutaneous (peripheral) pulse wave contour (PWC) changes in a 25-year-old and a 35-year-old normal pregnant patient showing the loss of downslope detail (decreased compliance). FIG. 1D shows cutaneous PWC recorded from a 19-year-old hypertensive pregnant patient which is similar to the carotid PWC recorded from hypertensive patients (3rd pulse, FIG. 1A), insofar that the 2nd maximum is higher than the first in both instances, thereby causing a prolongation of RET.
Peripheral vascular resistance is currently derived from invasive measurements of cardiac output and MAP. However, the invasive nature of this technique sharply limits its use in the prenatal patient.
Observations of the waveforms of noninvasively recorded carotid artery pulses indicate that the pulse wave contours change with advancing age and hypertension. In either situation, there is a decrease in arterial compliance and an increase in peripheral resistance. Moreover, Berne and Levy observed that "the heart is unable to eject its stroke volume into a rigid arterial system as rapidly as into a more complaint system. As compliance diminishes, peak arterial pressure occurs progressively later in systole."
From the foregoing, it is clear that although methods are available for assessing the determinants of arterial blood pressure, their clinical use is limited by the difficulties of noninvasevely obtaining a carotid artery pulse; even then the data can be acquired for only a few seconds.